1. Field of the Invention
This invention relates generally to circuit breakers with magnetic trip devices and more specifically to circuit breakers with means for adjusting the gap and spring bias on such magnetic trip devices.
2. Background
Circuit breakers provide protection for electrical systems from electrical fault conditions such as current overloads and short circuits. Typically, circuit breakers include a spring powered operating mechanism which opens electrical contacts to interrupt the current through the conductors on an electrical system in response to abnormal currents. The operating mechanism is unlatched by a trip bar which in turn is operated by a trip mechanism associated with each phase of the electrical system. Typically, the trip mechanism includes a magnetic trip device comprising a fixed magnetic structure energized by the current flowing through the conductor, and a movable armature which is attracted toward the stationary magnetic structure to operate the trip bar. The trip bar in turn unlatches the operating mechanism to open the electrical contacts in each phase of the electrical system. The moveable armature is biased away from the stationary magnetic structure by a spring thereby forming a gap between the armature and the stationary magnetic structure in the absence of an abnormal current.
Usually, means are provided for adjusting the level of current at which the magnetic trip device actuates the operating mechanism. Such adjustments can be made by varying the spring bias applied to the armature and/or mechanically adjusting the gap such as by varying the position of a threaded screw or cam against which the spring biases the armature. These adjustments permit fine tuning of the circuit breaker to assure that it will operate at the desired level of fault current. They can also be used to provide a range of settings at which the circuit breaker will trip. For instance, a circuit breaker may be rated to provide a range of trip settings between a low of say 500 amperes and a high of 1000 amperes.
U.S. Pat. No. 4,691,182 is an example of a circuit breaker having means for adjusting the spring bias and the gap for each pole of the breaker. The spring bias is adjusted individually for each pole by a rotatable cam which pivots a lever to adjust the bias applied to a rotatable armature by a tension spring. A threaded screw provides individual adjustment of the gap between the armature and the stationary magnetic structure. In this circuit breaker, it is not possible to separately adjust the gap at the high and low spring settings. The gap setting is the same for all settings established by the spring bias.
U.S. Pat. No. 4,630,019 discloses a circuit breaker in which the armature of the magnetic trip device is biased by a helically wound torsion spring. The spring bias may be adjusted by engaging one arm of the torsion spring in one of a number of slots in a support plate.
There are some circuit breakers which have been marketed in other countries in which means have been provided for simultaneously adjusting the spring bias on the armatures of the magnetic trip devices for all of the poles of the breaker. Some of these breakers also include means for adjusting the gap between the armature and the stationary magnetic structure through use of a threaded screw or a moveable cam device. However, to applicant's knowledge, none of these circuit breakers provide a means for independently adjusting the gap at the high and low spring settings.
While the common spring adjustment for all poles of a circuit breaker is a decided convenience in selecting the trip setting within the operating range of a breaker, the lack of individual gap adjustment at the high and low spring bias settings on presently available circuit breakers restricts the ability to provide accurate trip settings at both ends of the range of trip settings. For instance, by adjusting the spring bias to the high trip setting and adjusting the gap, it is possible to assure that the breaker will trip within a predetermined tolerance of the high setting. With a common spring adjustment for all poles and individual gap adjustments, this tolerance can be met for the high setting on all of the poles. However, when the spring bias is adjusted to the low trip setting, there is no assurance that the gap setting that was made for the high trip setting will provide a low trip setting within the desired tolerance.
There is a need therefore for a circuit breaker with a magnetic trip device in which gap adjustments may be individually made at high and low trip settings established by adjustment of the spring bias on the armature of the magnetic trip device. There is a preferred need for such a circuit breaker in which the spring bias can be adjusted simultaneously on all the poles of the circuit breaker while separate adjustment is provided for the high and low setting of each of the gaps individually.